ISE posture requirements and posture policy?

[baselzind]

I was checking posture in ISE and saw that posture requirement show "operating system", compliance module, and posture type but then again in posture policy they are mentioned again where I also set the requirement which I configured earlier. my question is if I choose these parameters in requirement ("operating system" , compliance module and posture type) why do i choose them again in posture policy? 

[Rob Ingram]

@baselzind 

The posture policy is a collection of the posture requirements, these are associated with the operating systems and/or ID groups. It's this posture policy that dictates to the client the posture requirements for the connection. Having the ability to differentiate between OS and/or ID groups in the posture policy allows the ability to match different requirements to the devices.